Sunday, 15 February 2015

Infidelity Among the Aardwolves?

This year's Synapsida survey of the species in a family of mammals is, of course, that of the dog family. I've already covered wolves and coyotes, jackals will be up next, and then, alongside a few others, you're going to see an awful lot of foxes. What you won't see any of are hyenas.

That's because hyenas, physical appearance notwithstanding, are not dogs. In fact, they're actually more closely related to cats than they are to dogs. (Although they're closer to mongooses than to either, for what it's worth). One of these days I may get round to a description of the hyena family, and how it's different to that of the dogs, but that won't be a terribly long series of posts, because there are only four living species.

There's the one everybody knows, which is the spotted or "laughing" hyena, and there's a couple of smaller, rarer hyenas related to it. And then, there's the aardwolf (Proteles cristata). Aardwolves are, to be honest, pretty strange animals, and there's a lot to be said about them, their feeding habits, and so on. This, however, is not that post. Because, yesterday being Valentine's Day and all, it's time to talk about mating behaviour.
There are a number of ways of classifying mating systems among mammals (or, indeed, other animals), but one of them considers there to be four basic patterns. Perhaps the most common in mammals is polygyny, where males mate with as many females as they can get away with. Polygynous species typically have males much larger than females, often with adornments such as horns that can both serve as signals of sexual fitness, and be handy when fighting off rivals. A common alternative is monogamy, in which a pair of animals stay faithful to one another, at least while they raise their young, and often for much longer. Here, where neither partner needs to be very concerned about the other straying, males and females are often of similar size and build.

Polyandry, in which females get to call all the shots, and mate with lots of males, is not so common, but there are certainly examples. Here, the male wants to ensure that the one female he (and everyone else) is mating with has his kids, rather than those of his rival, and this requires more subtle biological tactics than simply fighting other males - having particularly big testicles that produce large volumes of sperm being one such method. The fourth option, of course, is promiscuity, where everyone has sex with everyone else until they're all bored with it. This option tends to produce animals where the sexes are behaviourally and physically similar, on the grounds that they both face similar challenges.

So, which of these is the aardwolf? Aardwolves are basically small, slender hyenas that eat almost nothing but termites. Termites being not exactly the most abundant resource ever, this means that they spend quite a lot of time looking for food. Which is a bit of a nuisance when you have young to look after, so it really makes sense to have somebody helping out. As so often in this kind of situation, this results in monogamy. When you need two parents to look after the children, polygyny just isn't going to cut it, because the father has to hang around at least long enough to help his partner out.

Except that, as is the case with many other animals, it doesn't seem to be quite that simple. There are a number of reports of male aardwolves mating with females other than their ostensible partner, for instance. And a study a couple of years ago showed that the way male aardwolves arrange and travel between their dens is exactly what you'd expect of animals on the prowl for new sexual partners. Given that they really do need to help look after the female's young if they're to have much chance of surviving, this puts them in a conflicting situation: wanting to stay with their female, and also wanting to sleep around. If the social aspects of their mating habits - who they live with, in other words - differs from their actual sexual activity, which has the greater effect on other aspects of their behaviour?

And, come to that, why do they do it? If you stay around to help look after your partner's kids, but you don't know for certain that some other male hasn't done exactly what you'd do in the same situation, and snuck in for a quick one when you weren't looking... well, you may be wasting a lot of time raising someone else's children. The females aren't going to care, since they at least know the children are theirs, but it's a different matter for the males.

There have not been a huge number of studies on aardwolves, which are, to be honest, not among the best known animals in Africa. A new analysis of aardwolf behaviour in South Africa is also the first ever to take a look at their hormonal status over the year, and how that might be affecting, or be affected by, their mating habits.

Aardwolves mate during the dry season, probably during a relatively short window of opportunity, and we don't even know for sure that the females come into heat more than once a year. But, while much of the dry season presumably isn't also the "mating season", there should still be some differences in behaviour between that and the wet season, when adults are looking after their young. One of the things we can look at here is how the animals move around, as assessed by radio tagging them and examining the resulting GPS data.

It turns out that, as in most other mammals, males travel over a wider area than females do, although not by very much (4 to 5 km2, rather than 2 to 4 km2). Both sexes also range over a slightly wider area during the wet season than during the dry. Neither of these things is what you would expect of a polygamous mammal. If males want to mate with as many females as possible, they should travel over a much bigger area, in order that several females' territories will overlap with theirs. And, if anything, they should do it more during the dry season, when mating opportunities are available. Not only that, but both sexes make even use of their territories, wandering about in them freely; there is not, in other words, any evidence that the males prefer to hang around at their territory borders, which is where they're going to meet any neighbouring females.

Males do, it seems, scent mark their territories more than females do, which might be taken as advertising their presence, both to warn off rivals, and attract the attention of unfamiliar females. Except that, again, they do it even more during the wet season... which suggests that they're more concerned about protecting their young than they are about the possibility of their partner mating with strangers.

To evaluate hormone levels, the researchers sampled and analysed a great amount of aardwolf poo, looking for metabolites that would indirectly indicate the levels of four hormones in the blood. Three of these are, perhaps, obvious choices for this sort of study: the female sex hormones oestrogen and progesterone, and the male sex hormone epiandrosterone (which isn't quite the same thing as testosterone, but is functionally similar). The remaining one was the stress hormone cortisol. The idea here is that, if males compete for mates, then the ones who lose out should have higher cortisol levels during the mating season, something that's known to be the case in a number of polygynous species.

But, so far as we could tell, that wasn't happening here: stress hormone levels during the dry season were almost exactly the same in every individual examined. In other words, they were all doing equally well, and so presumably weren't competing. The two female hormones were slightly higher during the dry season, as one might expect, but male hormones were also higher, not just in the males, but in the females, too.

While mammals always have a small amount of hormones associated with the other sex in their bloodstreams, even when perfectly healthy, hyenas in general are a bit odd when it comes to this, in that the females have unusually high levels of "male" hormones. This seems to be related, at least in part, to their aggressive natures, and is most extreme in the spotted hyena (which also has the even more bizarre property that the females appear to possess a penis and scrotum). It's possible that these become higher in female aardwolves during the mating season because there is some competition among them to hold on to their mates, and a need for more aggressive behaviour as a result.

That aside, though, all the indications are that the aardwolves in this study were acting like monogamous animals, and that their sex and stress hormone levels largely matched that behaviour, too. So, even if they aren't entirely faithful, for the most part they seem geared to act as if they are. We've seen why that is - they need two stable parents to look after their young. This study cannot, however, show why they aren't quite as faithful in practice as their social lives would indicate they ought to be.

Maybe it's those high levels of male hormones, promoting aggression in both males and females, and leaving the former, in particular, with a high sex drive that doesn't quite mesh with the clear necessity to stand by your mate. Perhaps its a holdover from some relatively recent evolutionary change in their behaviour, since spotted hyenas are anything but monogamous. Sometimes biological desires and social practicalities come into conflict.